A day after Sri Lankan cricketers took to the field here wearing anti-pollution masks, Delhi's air turned fouler today with a sharp rise in the level of particulates since morning.

The concentration of the most dominant pollutants, PM2.5 and PM10, reached levels as high as 276 and 455 micrograms per cubic metre by 3 pm, according to the Central Control Room for Air Quality Management of the CPCB.

Pollution is considered severe plus or emergency when these readings are above 300 and 500 respectively.

The corresponding prescribed standards are 60 and 100.

The Air Quality Index (AQI), as recorded by the Central Pollution Control Board (CPCB), also reflected the rise in the levels of these ultrafine air-borne particulates.

The AQI was 390, on a scale of 500, around 3.30 pm. It is classified as 'very poor'. However, it is menacingly close to the 'severe' category.

Yesterday's 24-hour average was 351.

A 'very poor' AQI comes with the warning that people may develop respiratory illness on prolonged exposure while exposure to 'severe' air affects healthy people and seriously impacts those with existing respiratory or cardiovascular diseases.

An AQI between 0-50 is considered Good, 51-100 Satisfactory, 101-200 Moderate, 201-300 Poor, 301-400 Very Poor, and 401-500 Severe.

Pollution levels in Delhi usually remain in the 'very poor' category during December.

November witnesses extremely high levels of pollution as smoke from paddy residue burning reaches the city, aggravating the situation.

However, there are phases, when such spikes are seen even during December and January, mostly due to rapid drop in temperature and high moisture content, factors that trap particulates near the surface.

According to an IIT-Kanpur study, the sources of PM2.5 and PM10 during the winter months are: secondary particles like nitrates (25-30 per cent), vehicles (20-25 per cent), biomass burning (1726 per cent), municipal solid waste burning (9-8 per cent) and to a lesser extent soil and road dust.

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